With today's highly distributed communication systems, customers demand that the communication systems just work. To deal with this demand, communication providers build in redundancy in their networks to preserve communications when a network element fails. As new protocols evolve, such as the Session Initiation Protocol (SIP), providing redundant communication systems that will provide failover for already established communications can be, at times, challenging.
In cases where an existing communication session is established or is in the process of being initiated, trying to maintain the state of the existing communication session on a failed over system can be difficult. This can be due to various factors such as protocol requirements. For example, trying to redirect an existing SIP communication session to a failover system typically will be rejected by the failover system. This is due to the failover system not being able to recognize that the redirected communication is for an existing communication session.